The turbine blades, or airfoils, of axial flow gas turbine engines are directly exposed to the hot gas stream which exits the combustion section of the gas turbine engine. As combustor temperatures are increased to increase the performance of gas turbine engines, the temperature of the hot gas stream may exceed the critical design temperature limits determined by the airfoil material capability, maximum stress, and life requirement considerations. To prevent this, the external surface of the airfoils may be cooled by discharging cooling air from an internal cavity of the airfoil onto the external surface thereof through a plurality of small passages to create a film of cooling air. This film forms a boundary layer on the external surface of the airfoil downstream of each passage thereby providing a protective film of cool air between the hot gas stream and the external surface of the airfoil. The angle between the axis of the passage and the external surface of the airfoil, and the direction of the flow of the hot gas stream over the airfoil surface at the outlet of each passage are important factors which influence the effectiveness of the film cooling. Since film cooling effectiveness decreases rapidly with increasing distance from a particular cooling passage outlet, maintaining high film cooling effectiveness for as long a distance as possible over as large a surface area as possible is the main goal of airfoil film cooling.
It is well known in the art that the engine airfoils must be cooled using a minimum amount of cooling air, since the cooling air is working fluid which has been extracted from the compressor, and bleeding off working fluid downstream of the compressor reduces overall engine efficiency. Thus, airfoil designers face the problem of providing for cooling of all the engine airfoils using a maximum allowable cooling fluid flow rate. The amount of fluid which flows from the internal cavity of the airfoil into the hot gas stream through each individual cooling passage is controlled by the metering area--the minimum cross-sectional area--of the cooling passage. The metering area is typically located adjacent the intersection of the passage and the internal cavity. The combined metering areas of all of the cooling passages and orifices leading from the internal cavity of a given airfoil limits the total flow rate of coolant from the airfoil for a given pressure differential between the internal cavity and the external surface of the airfoil. The airfoil designer has the job of specifying the passage size and the spacing between passages, as well as the shape and orientation of the passages, such that all areas of the airfoil are maintained below critical design temperature limits. Ideally, designers of airfoils would like to bathe the entire external surface of the airfoil with a film of cooling air. However, cooling air leaving the passage exit usually forms a narrow strip of cooling film immediately downstream of each cooling passage outlet. This strip is typically as wide as the width of the passage outlet, that width being the dimension of the passage outlet that is perpendicular to the flow of the hot gas stream. Since each passage reduces the structural integrity of the airfoil to some extent, limitations must be placed on the number, size, and spacing of cooling passages. These limitations have resulted in gaps in the protective film and/or areas of low film cooling effectiveness that produce hot spots on the airfoils. Airfoil hot spots are one factor which limit the operating temperature of the engine.
What is needed is a means of providing improved film cooling effectiveness without increasing the amount of cooling air required and without significantly reducing the structural integrity of the airfoil.